1. Field of the Invention
The present invention generally relates to medical cardiac pacers, and more particularly, it relates to a pacer of the type which responds to the patient's metabolic demand and varies the decay rates in substantial similarity to the heart's normal behavior.
2. Description of the Prior Art
Early cardiac pacemakers provided a fixed-rate stimulation pulse generator that could be reset, on demand, by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex simulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
In recent years, single and dual chamber pacemakers have been developed which measure parameters which are directly or indirectly related to metabolic requirements (e.g., demand oxygenated blood) and vary the pacing rate in response to such parameters. Such measured parameters include, for example, physical activity of the body, right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, venous blood oxygen satuation, respiration, minute ventilation, and various pre and post-systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart. Such sensor-driven pacemakers have been developed for the purpose of restoring rate response to exercise in patients lacking the ability to increase rate adequately by exertion.
In general, a rate responsive pacemaker includes a sensor which produces an output that varies between a maximum sensor output level and a minimum sensor output level ("Sensor Output"), and a packing rate is provided by the pacemaker ("Pacing Rate") which typically varies as a linear or monotonic function ("f") of the sensor output, between a selectable lower pacing rate ("Lower Rate") and upper pacing rate ("Upper Rate"). Function f has a selectable slope (i.e., Pacing Rate change/Sensor Output Change) adjustable by means of an external programmer in conjunction with the Lower and Upper Rates. Thus, the Pacing Rate typically provided is equal to the pre-selected Lower Rate plus an increment which is a function of the measured Sensor Output, as follows: EQU Pacing Rate=Lower Rate+f(Sensor Output).
While this rate response technique provides a useful and workable system between the programmed parameters, the behavior of the pacemaker is complex and not often readily apprehended. Among these rate responsive pacemakers, those that measure the physical activity of the patient by means of a piezoelectric transducer have become popular among the various rate responsive pacemakers. Such an activity rate responsive pacemaker is described in U.S. Pat. No. 4,485,813 issued to Anderson et al.
Some temperature sensing pacemakers have employed relatively more complex functions to take into account the initial dip in temperature due to the onset of exercise. One such pacemaker is described in U.S. Pat. No. 4,719,920 issued to Alt.
Furthermore, the decay slope of conventional pacemakers do not approximate the heart's normal behavior, in that they are programmed to follow a curve based on a single time constant. This discrepancy between the normal heart deceleration function at the end of physiologic stresses, such as physical activity, and the conventional decay function has not been totally rectified by any pacemaker presently available on the market.
Wherefore, it is desirable to have a new cardiac pacemaker and method of pacing with activity or other rate responsive dependent parameters, for responding to the patient's metabolic demand and for varying the attack and decay pacing rates in harmony with the heart's normal behavior.